It is known that titanium is a metal which has advantageous features. In particular, it is light, substantially insensitive to corrosion, and has good mechanical properties. On the other hand, titanium has a serious drawback, in that it has very poor friction properties, and seizes when it is made to rub on itself or on another material, such as, for instance, steel.
Because titanium is essential to advanced industries, such as those dealing with space travel, many considerable efforts have been made to give it a good frictional strength at all costs.
Promising results have been obtained, in particular, through oxidation. As a matter of fact, it has been found that by coating titanium or titanium alloy parts with a coat of an oxide, most of the time based on TiO.sub.2, some frictional strength was obtained. To form said coat of oxide, various means have been used, such as anodic oxidation, heating under an oxidizing atmosphere in an enclosure, or the like. However, the results obtained were never completely satisfactory, due to either the coat flaking off during the friction, or scale being generated on the surfaces of the parts.
Some workers have tried to obviate such drawbacks by carrying out the treatment in a number of steps, for instance, by producing a layer of scale on the surface of the part to be treated through an oxidation in the oxygen, and then heating the coated part in argon in order that said coat of scale may become diffused within the titanium part. But such a method, which is difficult to work out on an industrial scale, has a further drawback in that it requires high treating temperatures, on the order of about 900.degree. C., which is much higher than the temperature generally accepted for annealing titanium, and long treatment time exceeding 24 hours.
In the area of surface treatments, there are many known methods applicable to many types of materials. The principal technologies known to applicants are electrolytic processes, and low-pressure treatments in alkaline baths or gaseous atmospheres. Each of these technologies presents advantages and disadvantages, either during processing or in obtaining essential parameters required for obtaining the desired surface layers. Some methods require high temperatures which are not compatible with the materials to be treated. For instance, aluminum alloys are damaged by high temperature treatment, and wear resistance of titanium alloys is decreased by the modification of its physical structure by exposure to high temperatures. Electrolytic methods require extremely precise surface preparation to obtain perfectly adherent layers, which are absolutely necessary for all mechanical parts submitted to frictional forces. Also, electrolytic processes produce thin TiO.sub.2 layers.
Among the prior art known to applicants are U.S. Pat. Nos. 3,322,577, and 3,779,816. U.S. Pat. No. 3,332,577, issued in 1967 to Smith, entitled "METHOD AND APPARATUS FOR THE CONTINUOUS PRODUCTION OF OXIDE COATINGS" relates to a low pressure treatment of aluminum to produce a single surface layer of Al.sub.2 O.sub.3 as a dielectric film in an electrolytic capacitor, or to a similar single layer from tantalum for the same purpose. Electron bombardment heating is used to limit the film thickness.
A single component layer is not suitable for a titanium friction surface.
U.S. Pat. No. 3,779,816, issued in 1973 to Mao, entitled "METHOD OF MAKING MOLD FOR FORMING OBJECTS" relates to the formation of a T.sub.i O.sub.2 (rutile) layer as a release agent for battery grid molds, by a constant flow, low pressure process.
As is known, T.sub.i O.sub.2 is very weak, especially in thin layers, and unsuitable for frictional surfaces.
The present invention provides a method adapted to be used commerically for oxidizing titanium and the alloys thereof, which avoids all the above drawbacks and gives extraordinarily good results, such as have never been obtained up to now in the field of resistance to seizing and wear.